Multiple suction variable capacity screw compressors are well known as shown in the Kocher U.S. Pat. No. 3,577,742, issued May 4, 1971. This type of a compressor has been further improved by using a slide valve to vary the capacity of the compressor from a minimum to a maximum level as shown in Edstrom U.S. Pat. No. 3,738,780, issued June 12, 1973. In compressors of this type the injection point for the secondary or high pressure vapor remains constant. The slide valve for the primary suction inlet is moved to change the capacity of the compressor from a maximum to a minimum producing a continuing change in the pressure of the intermediate vapor. At minimum capacity, the intermediate vapor pressure will be virtually at the same pressure level as the primary vapor pressure from the main evaporator. This continually changing intermediate vapor pressure level reduces the effect of the multiple suction arrangement such that maximum efficiency occurs only at maximum load conditions. There is virtually no increase in efficiency at minimum capacity because the pressures at the primary and secondary inlets are the same. Since the intermediate vapor pressure is continually varying in this type of a compressor, it is not suitable for use with intermediate evaporators. More specifically, a conventional multiple suction arrangement has the intermediate evaporator inlets located in the compressor casing around the circumference in an area where the slide valve is not located resulting in the openings always being in a fixed position so that, as the slide valve moves, the distance between the start of compression and the point at which the secondary vapor is injected is continually decreasing as the slide valve moves to the unloaded position. In fact, when the slide valve has moved to the point where the compressor is unloaded only 30%, the intermediate port is uncovered and the intermediate pressure is then reduced to the same pressure level as the main evaporator eliminating the beneficial effects of the constant intermediate pressure at a level above the main evaporator suction pressure. In some prior art patents, namely, Schibbye U.S. Pat. No. 3,314,597, Schibbye U.S. Pat. No. 3,342,089, Persson U.S. Pat. No. 3,756,753, Edstrom U.S. Pat. No. 3,734,653, and British Pat. No. 1,237,333, ports in the slide valve which have nothing to do with a high pressure suction are used for the injection of lubricating and cooling oil. This has no effect on the capacity of the compressor and does not provide for the increased capacity by being able to admit additional refrigerant vapor to the compressor through the high pressure suction port. These patents use a slide valve as a means for admitting cooling oil or refrigerant liquid for the cooling purposes and is a means of improving on the cooling and lubrication of a compressor. It has nothing to do with increasing the capacity of the compressor from the standpoint of refrigerant vapor compressed and rate of refrigerant circulated through the system.